1. Field of the Invention
The present invention relates to a method for producing a glass preform for use in fabrication of an optical fiber. More particularly, it relates to a method for producing a glass preform for use in fabrication of an optical fiber, particularly a single mode optical fiber having a zero dispersion wavelength at least in a wavelength band of 1.5 .mu.m.
2. Description of Prior Arts
An optical fiber made of quartz base glass has a minimum attenuation of light transmission in a wavelength range between 1.5 .mu.m and 1.6 .mu.m. In order to minimize the interval between two junctions in an optical fiber transmission system, light in a wavelength band of 1.5 .mu.m should be used. In this case, a single mode optical fiber is used rather than a multi-mode optical fiber to transmit information at a high transmission rate since the former has much wider transmission band than the latter. For transmitting information at a very high transmission rate, it is necessary to minimize an optical dispersion effect at the wavelength of light to be transmitted.
A currently used single mode optical fiber for a wavelength of 1.3 .mu.m is designed so that material dispersion and structural dispersion offset each other to make an optical dispersion sum zero.
In order to use the single mode optical fiber in the 1.5 micron wavelength band and minimize the optical dispersion in this wavelength band, there are two methods, one of which is to design a structure of an optical fiber so that the optical dispersion is zero in this wavelength band and the other of which is to use a light source having a very narrow spectral band width at a wavelength used in this wavelength band.
The present invention intends to minimize the optical dispersion in the 1.5 micron wavelength band by designing a structure of an optical fiber so that the optical dispersion is zero at least in this wavelength band.
A typical refractive index distribution of an optical fiber having such structure is shown in FIG. 1A, 1B or 1C. An optical fiber having the refractive index distribution of FIG. 1A or 1B has a low optical dispersion level in the 1.5 micron wavelength band and a wavelength band wider than it. In these cases, the optical fiber can be used at other wavelength and, further, it is advantageously used for wavelength multiplex transmission.
For the production of an optical fiber having the refractive index distribution of FIG. 1A, 1B or 1C, available are a modified chemical vapor deposition (MCVD) method and an outside vapor deposition (OVD) method. In these methods, layers of glass soot having different refractive indices are deposited in a radial direction on an inner or outer surface of a starting glass tube to provide a desired refractive index distribution and collapsing a tube-like glass preform into a rod-like glass preform.
An optical fiber having relatively good performance can be fabricated from the glass preform produced by the conventional methods. However, it is very difficult to precisely control the refractive index distribution since the domain in which the refractive index should be varied is a central portion of the fiber of about 10 .mu.m in diameter, which is much smaller than an outer diameter of the usual optical fiber of about 125 .mu.m. Furthermore, since an additive for adjusting the refractive index of glass, namely a dopant, tends to evaporate during collapsing the tube-like preform into the cylindrical rod in the conventional methods, the refractive index distribution in the central portion of the optical fiber is disadvantageously fluctuated.
By a conventional vapor phase axial deposition (VAD) method, it is rather difficult to produce a glass preform having plural maximum and minimum points in the refractive index distribution as shown in FIG. 1A, 1B or 1C.